This invention relates to a method and apparatus for accurately indicating the location of a spool in a hydraulic servo valve. More particularly, this invention relates to a linear Hall effect spool position indicator for electro hydraulic servo valves.
Two stage hydraulic servo valves have generally comprised a first stage valve element operated by any desired means such as an electrical torque motor or a pneumatic or hydraulic pilot valve and a second stage valve element having a spool axially movable within a casing by fluid pressure applied to one end or the other under control of the first stage valve element. Such valves have been typically controlled by a low power electrical or fluid command signal and in order to ensure precise accurate output from the device, it is necessary to know precisely the location of the second stage spool so as to accurately control its position in accordance with the first stage signal.
Various feedback mechanisms have been disclosed in the past including balanced beams, strain gauges and electro magnetic means actuated by displacement of the spool. For instance, in U.S. Pat. No. 4,585,029 to Harding there is shown a beam mounted for swinging movement with respect to the overall casing with one end of the beam being engaged by the spool and the other end of the beam carrying a Hall effect sensor. The Hall effect sensor is positioned between a magnet assembly housed in the casing so that as the beam is displaced by the spool, the other end is displaced within the magnetic field to create a signal proportional to the displacement to indicate the position of the spool. The beam is pivotally mounted on leaf springs with one end in the air gap of the magnetic circuit and the other end in a recess of the spool. This design has proven satisfactory for certain applications, but the beam construction is very sensitive to inaccuracies caused by spurious influences such as temperature, vibration, impact and the like. In addition, this construction must be very accurate and precise, and is therefore costly to effect. The mechanical nature of the construction also dictates a relatively bulky mechanical/physical structure that is difficult and costly to manufacture.